Computer Networks. The Socket API (Project 1) & Traceroute (HW 1)

Computer Networks

The Socket API (Project 1) & Traceroute (HW 1)

Originally By David Wetherall (djw@)

Outline

Ø

Administrivia

Ø

Project 1 and Socket API

Administrivia

Computer Networks 3

Ø

Sections will be recorded

Ø

HW1 & Traceroute Experiment due tomorrow

Ø

HW2 due Jan 21

st

_{(next Friday)}

Network-Application Interface

Ø

Defines how apps use the network

Ø Application Layer APIs

Ø Lets apps talk to each other

Ø hides the other layers of the network

app

app

Project 1

Computer Networks 5

Ø

Simple Client

Ø Send requests to attu server Ø Wait for a reply

Ø Extract the information from the reply Ø Continue…

Ø

Simple Server

Ø Server handles the Client requests Ø Multi-threaded

Client

Time

_Server

Computer Networks

request

reply

extract

information

request

Continue…

Project 1

Ø

This is the basis for many apps!

Ø File transfer: send name, get file Ø Web browsing: send URL, get page Ø Echo: send message, get it back Ø Let’s see how to write this app …

Socket API (Generalized)

Computer Networks 7

Ø Simple application-layer abstractions (APIs) to use the network Ø The network service API used to write all Internet

applications

Ø Part of all major OSes and languages; originally Berkeley (Unix) ~1983

Ø Two kinds of sockets

Ø Streams (TCP): reliably send a stream of bytes

Socket API (2)

Ø

Sockets let apps attach to the local network at different ports

Ø

Ports are used by OS to distinguish services/apps using internet

Socket Port 1

Socket Port 2

Socket API (3)

Computer Networks https://docs.oracle.com/javase/8/docs/api/java/net/Socket.html https://docs.oracle.com/javase/8/docs/api/java/net/ServerSocket.html 7

Primitive

Meaning

SOCKET

Create a new communication endpoint

BIND

Associate a local address (port) with a socket

LISTEN

Announce willingness to accept connections; (give

_{queue size)}

ACCEPT

Passively establish an incoming connection

CONNECT Actively attempt to establish a connection

SEND

Send some data over the connection

RECEIVE

Receive some data from the connection

Using TCP Sockets (2)

Client (host 1)

Time

_{Server (host 2)}

request

reply

disconnect

1

1

2

3

connect

Using TCP Sockets (3)

Client (host 1)

Time

_{Server (host 2)}

5: connect*

1: socket

9: send

6: recv*

7: send

8: recv*

11: close

request

Computer Networks 11

reply

disconnect

connect

*= call blocks

1: socket

2: (bind)

3: (listen)

4: accept*

10: recv*

12: close

Using UDP Sockets

Client (host 1)

Time

_{Server (host 2)}

5: connect*

1: socket

9: sendto

6: recvfrom*

7: sendto

8: recvfrom*

request

reply

disconnect

connect

*= call blocks

1: socket

2: (bind)

3: (listen)

4: accept*

10: recvfrom*

Client Program (outline)

Computer Networks 13

socket()

// make socket

getaddrinfo() // server and portname

// www.example.com:80

connect() // connect to server [block]

…

send()

recv()

…

close()

// send request

// await reply [block]

// do something with data!

Server Program (outline)

socket() // make socket

getaddrinfo() // for port on this host // associate port with socket // prepare to accept connections // wait for a connection [block] bind() listen() accept() … recv() … send() close()

// wait for request [block] // send the reply

Java Examples with Socket & ServerSocket

Computer Networks 15

Ø

Server

• http://cs.lmu.edu/~ray/notes/javanetexamples/ • https://docs.oracle.com/javase/tutorial/net working/datagrams/clientServer.html • https://docs.oracle.com/javase/tutorial/net working/sockets/index.html

ServerSocket listener = new ServerSocket(9090);

try{

while (true) {

Socket socket = listener.accept();

try{ socket.getInputStream(); } finally { socket.close(); } } } finally { listener.close(); }

Socket socket = new Socket(server,9090);

out=

new PrintWriter(socket.getOutputStream(), true); socket.close();

Traceroute

Ø

Apps talk to other apps with no real idea of what is inside

the network

Ø This is good! But you may be curious …

Ø

Peeking inside the Network with Traceroute

app

app

Traceroute

Ø Widely used command-line tool to let hosts peek inside the network

Ø On all OSes (tracert on Windows) Ø Developed by Van Jacobson ~1987

Ø Uses a network-network interface (IP) in ways we will explain later

Computer Networks 15

: Credit: Wikipedia (public domain)

Traceroute

Ø

Probes successive hops to find network path

Ø

Core mechanism: Time-To-Live(TTL)

Ø

TTL == 0?

Discard data, error (ICMP) report to sender

Continue with TTL-1

Local

Traceroute

. . .

Local

Host

1 hop

_{2 hops}

3 hops

N-1 hops

N hops

Remote

Host

Computer Networks 19

Using

Traceroute

21

Using Traceroute

Router settings

affect results

Using Traceroute (2)

Ø

ISP names and places are educatedguesses

My computer

www.uw.edu

(www1.cac.washington.edu)

Level3 pnw-gigapop

UW

6 hops

1 hop

3 hops

. . .

tde Telefonica

3 hops

4 hops

100 ms

180 ms

NYC

San Jose

Computer Networks 23

UW

>200 ms

Seattle

Home

1 hop

END

© 2013 D. Wetherall